The biggest speculation in The Big Bang Theory and Young Sheldon fandom is that Leonard Hofstadter, Sheldon's best friend, is potentially dead. This theory took hold of the fans when Leonard was the only character from the gang of friends who was never mentioned in Sheldon's look back at his childhood through the Young Sheldon spin-off.
The total lack of mention sent the fandom into a flurry of speculations and theories, after which they arrived at the conclusion that maybe Leonard was dead, and because Sheldon was not that adept at dealing with his emotions, he just never mentions Leonard.
However, the latest spin-off from this universe, centering around Stuart, titled Stuart Fails to Save the Universe, might just be the update that the fandom needed to dispel this theory.
How did the Leonard is Dead theory originate after Young Sheldon

One of the things that made Young Sheldon as special and successful as it ended up being was Jim Parsons' voice-overs as the adult version of Iain Armitage's Sheldon. This addition, along with the heartwarming characters and story, led to giving the spin-off its own identity rather than being known as The Big Bang Theory spin-off.
Another characteristic that made Young Sheldon special for The Big Bang Theory fans was the mentions of the friend gang in Parsons' voiceovers or the surprise visit from one of them in the voiceovers.
The series finale for Young Sheldon featured Mayim Bialik as an older version of Amy, Sheldon's wife. The show also featured Howard for voice cameos, and it provided updates on Sheldon's friends and Penny. However, Leonard Hofstadter, Sheldon's best friend and Penny's husband, is never mentioned or brought up in the show.
This lack of update or mention, even though Leonard was the closest to Sheldon, led the fandom to believe that Leonard was dead. According to the theory, Sheldon has never been great at handling heavy emotions, so it could well be speculated that he avoids talking or thinking about Leonard to avoid having to deal with the emotional weight of his death.
How this Young Sheldon theory is being dispelled by Stuart Fails to Save the Universe

The main reason why the fandom came to the conclusion that Leonard might be dead is that he was never mentioned in the voiceovers, and there were no updates about his whereabouts, even when Penny, his wife, is mentioned in the voiceovers.
However, the latest spin-off, Stuart Fails To Save The Universe, might just be responsible for putting these theories to rest. The show is set after the events of The Big Bang Theory but before the events of the Young Sheldon finale, where Sheldon and Amy make a cameo.
The logline for the show states that:
"Comic book store owner Stuart Bloom (Sussman) is tasked with restoring reality after he breaks a device built by Sheldon and Leonard, accidentally bringing about a multiverse Armageddon."
This mention of Leonard in the logline for the show proves that Leonard continues to be an important part of Sheldon's life, professionally and personally, formally putting all the conspiracy theories to rest.
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